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<a name="Suicidal-exception-handling"></a>
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<hr size="1">
<a name="Suicidal-exception-handling-1"></a>
<h4 class="subsubsection">3.9.3.3 Suicidal exception handling</h4>

<p>An inconvenient characteristic of some external library functions is
to terminate the program rather than returning an error status to the
caller for routine events such as a failure of memory allocation.
Although in many cases there is no simple workaround for this
behavior, memory allocation failures at least can be detected and
preventive action taken by using the functions described in this
section.
</p>
<p>The general approach is to use memory management functions from
&lsquo;<tt>mwrap.h</tt>&rsquo; as described previously (<a href="Memory-leaks.html#Memory-leaks">Memory leaks</a>), while
additionally registering a return destination for a non-local jump to
<a name="index-non_002dlocal-jumps"></a>
be taken in the event of a memory overflow. The jump is taken when an
external library function calls <code>malloc</code> or <code>realloc</code>
unsuccessfully.  The jump avoids passing control back to the library
function, thereby denying it the opportunity to abort, but restores
the context to that of the jump destination almost as if the library
function and all of its intervening callers had returned normally.
</p>
<p>The interface is similar to that of the standard <code>setjmp</code>
<a name="index-setjmp"></a>
function defined in the system header file <code>setjmp.h</code>, and in
fact is built on it, but differs in that the client module does not
explicitly refer to jump buffers. Instead, the <code>mwrap</code> module
internally maintains a stack of return destinations.
</p>
<p>If a jump is taken, it always goes to the most recently registered
destination. It may revert to the previously registered destination
only when the current one is cleared. This organization provides the
necessary flexibility for multiple clients and recursion, but it
necessitates a protocol whereby each registration of a destination
must be explicitly cleared exactly once.
</p>
<p>The following functions implement these two features.
</p>
<dl>
<dt><a name="index-avm_005fsetjmp"></a><u>Function:</u> int <b>avm_setjmp</b><i> ()</i></dt>
<dd><p>This function specifies the point to which control will pass by a
non-local jump if there is insufficient memory to complete a
subsequent <code>malloc</code> or <code>realloc</code> operation. Only the
operations that take place while memory is being managed due to
<code>avm_manage_memory</code> are affected (<a href="Memory-leaks.html#Memory-leaks">Memory leaks</a>).
</p>
<p>The function returns zero when it is called normally and successfully
registers the return point.
</p>
<p>It returns a non-zero value when it has been entered by a non-local
jump (i.e., when <code>malloc</code> or <code>realloc</code> has reported
insufficient memory while memory management is active), or when the
return point could not be successfully registered due to insufficient
memory. The client need not distinguish between these two cases,
because both correspond to memory overflows and the destination must
be cleared by <code>avm_clearjmp</code> regardless.
</p>
<p>When a non-zero value is returned due to this function being reached
by a non-local jump, it has the side effects of reclaiming all managed
memory by calling <code>avm_free_managed_memory</code> and disabling memory
management by calling <code>avm_dont_manage_memory</code>.
</p></dd></dl>

<dl>
<dt><a name="index-avm_005fclearjmp"></a><u>Function:</u> void <b>avm_clearjmp</b><i> ()</i></dt>
<dd><p>This function cancels the effect of <code>avm_setjmp ()</code> by preventing
further non-local jumps to its destination if the destination was
successfully registered, or by acknowledging unsuccessful registration
otherwise. It should be called before exiting any function that calls
<code>avm_setjmp ()</code> or anomalous results may ensue.
</p></dd></dl>

<p>The memory management functions <code>avm_manage_memory</code> and
<code>avm_dont_manage_memory</code> can be useful with or without
<code>avm_setjmp</code>, depending on how much of a workaround is needed for
a given library. If a library does not abort on memory overflows,
there is no need to use <code>avm_setjmp</code>, while it may still be
appropriate to use the other functions against memory leaks.
</p>
<p>Calling <code>avm_clearjmp</code> is particularly important if a client
module with memory management that doesn&rsquo;t use <code>avm_setjmp</code> is
invoked subsequently to one that does, so that memory overflows in the
latter won&rsquo;t cause an attempted jump to a stale destination.
</p>
<p>A further complication that arises from careful consideration of these
issues is the situation of a client module that does not intend to use
<code>avm_setjmp</code> but is called (perhaps indirectly) by one that
does. The latter will have registered a return destination that
remains active and valid even if the former refrains from doing so,
thereby allowing a branch to be taken that should have been prevented.
Although it is an unusual situation, it can be accommodated by the
following function.
</p>
<dl>
<dt><a name="index-avm_005fsetnonjump"></a><u>Function:</u> void <b>avm_setnonjump</b><i> ()</i></dt>
<dd><p>This function temporarily inhibits non-local jumps to destinations
previously registered by <code>avm_setjmp</code> until the next time
<code>avm_clearjmp</code> is called. Thereafter, any previously registered
destinations are reinstated.
</p></dd></dl>

<p>A sketch of how some of these functions might be used to cope with
library functions that would otherwise terminate the program in the
event of a memory overflow is shown below. The GNU <code>libc</code>
<a name="index-non_002dlocal-jumps-1"></a>
reference manual contains a related discussion of non-local jumps.
</p>
<table><tr><td>&nbsp;</td><td><pre class="example">#include &lt;avm/mwrap.h&gt;
...

int
function foobar (foo, bar)
...
{
char *my_data;

  my_data = (char *) malloc (100);
  if (avm_setjmp () != 0)
    {
      avm_clearjmp ();
      avm_turn_on_stdout ();       /* reaching here */
      free (my_data);              /* means malloc  */
      return ABNORMAL_STATUS;      /* failed below  */
    }
  avm_turn_off_stdout ();
  avm_manage_memory ();
  ...
  call_library_functions (foo, bar);    /* may jump */
  ...                                   /* to above */
  avm_free_managed_memory ();
  avm_turn_on_stdout ();
  avm_clearjmp ();
  free (my_data);            /* reaching here means */
  return OK_STATUS;          /* jumping wasn't done */
}
</pre></td></tr></table>

<p>Portability issues with these functions are not well known at this
<a name="index-portability-1"></a>
writing. If the configuration script for <code>avram</code> fails to detect
the required features in <code>setjmp.h</code> on the host system,
conditional compilation directives will disable the functions
<code>avm_setjmp</code>, <code>avm_clearjmp</code>, and <code>avm_setnonjmp</code>.
However, it may still be possible for the other <code>avm_</code>* memory
management functions to be configured.
</p>
<p>If <code>setjmp</code> is not configured, the <code>avm_setjmp</code> function
is still callable but will always return a value of zero, and will
provide no protection against external library functions aborting the
program. The other two will perform no operation and return.
</p>
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